Iq-imbalance

ABSTRACT

The invention relates to a method for correcting an IQ-imbalance (In-phase and Quadrature) of an IQ-based direct conversion receiver ( 200 ). In the method a group of radio frequency pilot signals are received in the direct conversion receiver ( 200 ). They are conveyed to an in-phase branch and a quadrature-phase branch of the receiver ( 200 ) and mixed, in the analogue domain, to form a baseband in-phase (I) and quadrature-phase (Q) signal components. The signal componets are conveyed to a digital demodulator ( 210 ) which detects the IQ-phase imbalance of the direct conversion receiver ( 200 ) by analysing at least one of the baseband in-phase (I) and the quadrature-phase (Q) signal components. In the method, the detected IQ-imbalance is corrected, in the analogue domain of the direct conversion receiver ( 200 ), to achieve a 90 degrees phase difference between a future baseband in-phase (I) signal component and a future baseband quadrature-phase (Q) signal component.

[0001] The invention relates to correcting IQ-imbalance (In-phase andQuadrature) of a direct conversion receiver.

[0002] Digital broadcasting systems, such as digital television systems,are under development, and it is intended that, eventually, they willreplace analogue broadcasting systems. This is, among other things,because of their ability to provide new types of services and betterquality of service capabilities compared to the analogue broadcastingsystems.

[0003] One of the digital broadcasting systems currently being understandardisation by the European Telecommunications Standards Institute(ETSI) is the Digital Video Broadcasting (DVB) system. In the DVB systemdigital video is broadcast using satellite, cable and/or terrestrialinfrastructures.

[0004] The DVB system using terrestrial infrastructure is defined as theDVB-T (DVB-Terrestrial) system. In DVB-T transmission digital data ismodulated onto a radio frequency (RF) signal. The modulation method usedis the COFDM (Coded Orthogonal Frequency Division Multiplex) modulation.The modulated DVB-T signal is transmitted from a DVB-T transmitter. Thetransmission occurs over a DVB-T radio channel. The transmitted DVB-Tsignal is received at a DVB-T receiver. The DVB-T receiver demodulatesthe received DVB-T signal in order to regenerate the digital data. Thedigital data may contain, for example, an MPEG-2 (Moving Picture ExpertsGroup) coded video stream.

[0005] With terrestrial digital video broadcasting it is possible toachieve a good quality data transfer even if the DVB-T receiver ismobile.

[0006] Wireless mobile communication devices are expected to belightweight and small-sized. That is why the use of a direct conversionarchitecture in the DVB-T receiver is in many cases more desirable than,for example, the use of a superheterodyne architecture.

[0007] In a direct conversion receiver a radio frequency analogue DVB-Tsignal, transmitted from a DVB-T transmitter and received at a DVB-Treceiver, is converted directly from the radio frequency band tobaseband I- and Q-signal components in the DVB-T receiver, in which “I”refers to an in-phase signal component of the DVB-T signal and “Q”refers to a quadrature-phase signal component of the DVB-T signal. Inpractise, the conversion is performed by splitting the received radiofrequency DVB-T signal into two substantially identical signals, mixingone of the signals with a local oscillator signal to form the basebandI-signal component and mixing the other of the signals with a phaseshifted local oscillator signal to form the baseband Q-signal component.The phase shifting of the local oscillator signal is performed in aphase shifter the amount of the phase shift being 90 degrees. The phasedifference between the signals of an I-branch along which the I-signalcomponent travels in the DVB-T receiver and a Q-branch along which theQ-signal component travels in the DVB-T receiver is thus exactly 90degrees in an ideal case.

[0008] The COFDM modulation method that is used in the DVB-T system is amulticarrier modulation method. This means that, in the DVB-Ttransmitter, digital data to be transmitted is split into severalcomponents which are transmitted over separate carrier signals. In oneof the operational modes of the DVB-T system, for example, a DVB-Tchannel (the bandwidth of which is 8 Mhz) contains 6816 carriers (alsoreferred to as “subcarriers”). The carriers themselves are modulatedusing different level QAM (Quadrature Amplitude Modulation)constellations.

[0009]FIG. 1 shows a constellation diagram showing 64-QAM constellationpoints. The horizontal axis, that is the I-axis, indicates the amplitudeof the I-signal component of the DVB-T signal and the vertical axis,that is the Q-axis, indicates the amplitude of the Q-signal component ofthe DVB-T signal. In FIG. 1, the units of the I-axis and the Q-axis arearbitrary units. The I-axis and the Q-axis define an IQ-plane. Eachconstellation point in the IQ-plane corresponds to a transmitted bitsequence. The constellation point (I=3,Q=5), for example, corresponds toa transmitted bit sequence 001011.

[0010] It is important that the phase difference between the signals ofthe I-branch and the Q-branch is 90 degrees because if it differs from90 degrees there exists IQ-imbalance (more particularly IQ-phaseimbalance), and the probability that the transmitted bits are notdetected correctly in the DVB-T receiver rises. The more complex QAMmodulation is used the more sensitive the bit detection is to theIQ-imbalance.

[0011] The DVB-T system is a broadband system using a large variety offrequencies. If, for example, the UHF (UltraHigh Frequency) band is usedwith a channel width of 8 MHz, the used frequency band reaches from 470MHz to 862 MHz. It is difficult and expensive to manufacture a phaseshifter that would perform a stable 90 degrees phase difference for theI- and Q-branches in the whole region of the used frequency band,especially taking into consideration the small size requirement of thewireless mobile communication devices.

[0012] According to a first aspect of the invention there is provided amethod for correcting an IQ-imbalance of an IQ-based direct conversionreceiver the method comprising:

[0013] receiving a radio frequency signal in the direct conversionreceiver;

[0014] conveying, in an analogue domain of the direct conversionreceiver, the received radio frequency signal to an in-phase branch ofthe direct conversion receiver and to a quadrature-phase branch of thedirect conversion receiver;

[0015] mixing, in the analogue domain of the direct conversion receiver,the radio frequency signal of the in-phase branch with a first mixingsignal to form a baseband in-phase signal component and mixing the radiofrequency signal of the quadrature-phase branch with a second mixingsignal to form a baseband quadrature-phase signal component;

[0016] conveying the baseband in-phase signal component and the basebandquadrature-phase signal component to a digital demodulator;

[0017] detecting, in the digital demodulator, the IQ-imbalance of thedirect conversion receiver by analysing at least one of the basebandin-phase and the quadrature-phase signal components;

[0018] correcting the IQ-imbalance, detected in the digital demodulator,in the analogue domain of the direct conversion receiver to achieve a 90degrees phase difference between a future baseband in-phase signalcomponent and a future baseband quadrature-phase signal component.

[0019] According to a second aspect of the invention there is providedan IQ-based direct conversion receiver for correcting an IQ-imbalancethe direct conversion receiver comprising:

[0020] a radio frequency part for receiving a radio frequency signal inthe direct conversion receiver;

[0021] an in-phase branch and a quadrature-phase branch, in an analoguedomain of the direct conversion receiver, for conveying the receivedradio frequency signal in the in-phase branch and the quadrature-phasebranch;

[0022] a first mixer for mixing, in the analogue domain of the directconversion receiver, the radio frequency signal of the in-phase branchwith a first mixing signal to form a baseband in-phase signal componentand second mixer for mixing, in the analogue domain of the directconversion receiver, the radio frequency signal of the quadrature-phasebranch with a second mixing signal to form a baseband quadrature-phasesignal component;

[0023] a digital demodulator adapted to receive the baseband in-phasesignal component and the baseband quadrature-phase signal component, thedigital demodulator being adapted to detect the IQ-imbalance of thedirect conversion receiver by analysing at least one of the basebandin-phase and the quadrature-phase signal component;

[0024] means for correcting the IQ-imbalance, detected in the digitaldemodulator, in the analogue domain of the direct conversion receiver toachieve a 90 degrees phase difference between a future baseband in-phasesignal component and a future baseband quadrature-phase signalcomponent.

[0025] According to a third aspect of the invention there is provided acommunication device comprising an IQ-based direct conversion receiverfor correcting an IQ-imbalance the direct conversion receivercomprising:

[0026] a radio frequency part for receiving a radio frequency signal inthe direct conversion receiver;

[0027] an in-phase branch and a quadrature-phase branch, in an analoguedomain of the direct conversion receiver, for conveying the receivedradio frequency signal in the in-phase branch and the quadrature-phasebranch;

[0028] a first mixer for mixing, in the analogue domain of the directconversion receiver, the radio frequency signal of the in-phase branchwith a first mixing signal to form a baseband in-phase signal componentand second mixer for mixing, in the analogue domain of the directconversion receiver, the radio frequency signal of the quadrature-phasebranch with a second mixing signal to form a baseband quadrature-phasesignal component;

[0029] a digital demodulator adapted to receive the baseband in-phasesignal component and the baseband quadrature-phase signal component, thedigital demodulator being adapted to detect the IQ-imbalance of thedirect conversion receiver by analysing at least one of the basebandin-phase and the quadrature-phase signal component;

[0030] means for correcting the IQ-imbalance, detected in the digitaldemodulator, in the analogue domain of the direct conversion receiver toachieve a 90 degrees phase difference between a future baseband in-phasesignal component and a future baseband quadrature-phase signalcomponent.

[0031] According to a fourth aspect of the invention there is provided asystem comprising a transmitter and an IQ-based direct conversionreceiver for correcting an IQ-imbalance, the transmitter comprising amodulator for transmitting a radio frequency signal to the directconversion receiver the direct conversion receiver comprising:

[0032] a radio frequency part for receiving the radio frequency signalin the direct conversion receiver;

[0033] an in-phase branch and a quadrature-phase branch, in an analoguedomain of the direct conversion receiver, for conveying the receivedradio frequency signal in the in-phase branch and the quadrature-phasebranch;

[0034] a first mixer for mixing, in the analogue domain of the directconversion receiver, the radio frequency signal of the in-phase branchwith a first mixing signal to form a baseband in-phase signal componentand second mixer for mixing, in the analogue domain of the directconversion receiver, the radio frequency signal of the quadrature-phasebranch with a second mixing signal to form a baseband quadrature-phasesignal component;

[0035] a digital demodulator adapted to receive the baseband in-phasesignal component and the baseband quadrature-phase signal component, thedigital demodulator being adapted to detect the IQ-imbalance of thedirect conversion receiver by analysing at least one of the basebandin-phase and the quadrature-phase signal component;

[0036] means for correcting the IQ-imbalance, detected in the digitaldemodulator, in the analogue domain of the direct conversion receiver toachieve a 90 degrees phase difference between a future baseband in-phasesignal component and a future baseband quadrature-phase signalcomponent.

[0037] Embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings in which:

[0038]FIG. 1 shows a constellation diagram showing 64-QAM constellationpoints;

[0039]FIG. 2 shows the functional blocks of a DVB-T receiver accordingto a preferred embodiment of the invention;

[0040]FIG. 3 shows a detail of the DVB-T receiver of FIG. 2;

[0041]FIG. 4 shows a constellation diagram of 2-PSK modulation; and

[0042]FIG. 5 shows a mobile communication device according to theinvention.

[0043] The FIG. 1 has already been described in the foregoing.

[0044] In the following, a DVB-T system, according to a preferredembodiment, will be described. The system comprises a DVB-T transmitterand a DVB-T receiver. The DVB-T transmitter functions in a manner knownto a person skilled in the art. It comprises a COFDM modulator formodulating digital data to be broadcast to the DVB-T receiver.

[0045]FIG. 2 shows the DVB-T (broadband) receiver according to thepreferred embodiment of the invention. The DVB-T receiver 200 is locatedin a mobile communication device so as to form a portable wirelesshand-held device suitable for DVB-T operation. In addition to the DVB-Treceiving capability the mobile communication device may have a cellularnetwork capability in order to perform interactive communication with acellular network such as a cellular telephone network. The device may becalled a media terminal or a media screen.

[0046] The DVB-T receiver 200 functions generally according to a wellknown direct conversion principle.

[0047] In the preferred embodiment of the invention, digital data whichhas been COFDM modulated into an analogue radio frequency DVB-T signaland which has been sent from a DVB-T transmitter and received via anintegral antenna (not shown) of the DVB-T receiver 200 is conveyed to alow noise amplifier 201 of the DVB-T receiver 200. In the low noiseamplifier 201 the DVB-T signal is amplified in order to raise the powerlevel of the received DVB-T signal.

[0048] The amplified DVB-T signal is split into two substantiallyidentical signals one of which is conveyed to a first adjustableamplifier 202 and the other of which is conveyed to a second adjustableamplifier 203. The signal branch along which the first adjustableamplifier 202 resides is called a Q-branch and the signal branch alongwhich the second adjustable amplifier 203 resides is called an I-branch.

[0049] The first adjustable amplifier 202 amplifies the signaltravelling along the Q-branch and the second adjustable amplifier 203amplifies the signal travelling along the I-branch. In the Q-branch theamplified signal is conveyed to a first input of a first down conversionmixer 204 and in the I-branch the amplified signal is conveyed to afirst input of a second down conversion mixer 205.

[0050] A local oscillator 206 generates a local oscillator signal. Thelocal oscillator signal is conveyed to a second input of the second downconversion mixer 205. Additionally, the local oscillator signal isconveyed to an adjustable phase shifter 207 which shifts the phase ofthe local oscillator signal by 90 degrees. The phase shifted localoscillator signal is conveyed to a second input of the first downconversion mixer 204.

[0051] The first down conversion mixer 204 mixes the Q-branch signalcoming from the first adjustable amplifier 202 with the phase shiftedlocal oscillator signal in order to generate a baseband Q-signalcomponent. The baseband Q-signal component is conveyed to a first lowpass filter 208 which low pass filters the baseband Q-signal componentof the DVB-T signal. The first low pass filter 208 is a low pass filterof a fixed pass band, the width of which is substantially the same asthe width of one DVB-T channel divided by two. The pass band of thefirst low pass filter 208 has steep edges so as to strongly attenuatethe frequency components that lie outside the pass band. From the firstlow pass filter 208 the Q-signal component is conveyed to a COFDMdemodulator 210 for digital demodulation.

[0052] The second down conversion mixer 205 mixes the I-branch signalcoming from the second adjustable amplifier 203 with the localoscillator signal in order to generate a baseband I-signal component.The baseband I-signal component is conveyed to a second low pass filter209 which low pass filters the baseband I-signal component of the DVB-Tsignal. The second low pass filter 209 is a low pass filter of a fixedpass band, the width of which is substantially the same as the width ofone DVB-T channel divided by two. The pass band of the second low passfilter 209 has steep edges so as to strongly attenuate the frequencycomponents that lie outside the pass band. From the second low passfilter 209 the I-signal component is conveyed to the COFDM demodulator210 for digital demodulation.

[0053] The COFDM demodulator 210 is a digital demodulator block whichdemodulates the received COFDM modulated DVB-T signal so as toregenerate the originally transmitted digital data. From the COFDMdemodulator 210 the digital data may be conveyed, for example, to anMPEG-2 decoder, to an IP (Internet Protocol) stack or to anotherappropriate functional block for further processing.

[0054]FIG. 3 shows basic blocks of the COFDM demodulator 210. Those arean analogue signal processing block 301, an OFDM (Orthogonal FrequencyDivision Multiplex) demodulator block 302 and a FEC (Forward ErrorCorrection) channel decoder 303.

[0055] A first analogue-to-digital converter 331 of the analogue signalprocessing block 301 converts the Q-signal component from analogue formto digital form. A second analogue-to-digital converter 332 of theanalogue signal processing block 301 converts the I-signal componentfrom analogue form to digital form.

[0056] The digital I- and Q-signal components are conveyed to the OFDMdemodulator block 302 which performs resampling (in block 333) and FastFourier Transform (FFT, block 334) operations on the I- and Q-signalcomponents in order to generate a frequency domain DVB-T signal. Achannel estimation and correction block 335 of the OFDM demodulatorblock 302 determines, based on particular pilot signals comprised in theDVB-T signal, a transfer function H(f) of the DVB-T (receiving) channelin use, and based on the transfer function H(f), corrects the effectsthat the transmission path causes to the DVB-T signal. The OFDMdemodulator block 302 finally generates (in block 336), based on thecorrected DVB-T signal, soft decisions about the transmitted bits, thatis it generates probabilities about the transmitted bits.

[0057] The soft decisions are conveyed to the FEC channel decoder 303which performs a two-phase error correction. In the first phase aViterbi decoder 337 derives based on the soft decisions, usingredundancy inserted by a convolutional encoder of the DVB-T transmitter,the convolutional encoder input bit stream which most probably has beentransmitted from the DVB-T transmitter. The derived bit stream isconveyed to a Reed-Solomon decoder 338. The Reed-Solomon decoder 338performs the second phase of the error correction, that is the framedecoding. The frame decoded bit stream is conveyed to a descramblingblock 339 which performs descrambling operations in order to regeneratethe originally transmitted digital data.

[0058] It is apparent to a person skilled in the art that the blocks301-303 may comprise sub-blocks other than shown in FIG. 3 thosesub-blocks being apparent for a person skilled in the art. Block 301,for example, may contain an automatic gain control (AGC) for thereceived DVB-T signal. The AGC block may control with one or more AGCfeedback control signals the gain of the first and second adjustableamplifiers 202 and 203 so as to optimise the received DVB-T signalvoltage level for the analogue-to-digital conversion of the I- andQ-signal components. Block 303, for example, may contain additionalblocks for performing deinterleaving operations.

[0059] In one implementation of the DVB-T receiver 200 the blocks 201 to205 and 207 to 209 are implemented in one integrated circuit (IC) whichmay be called a direct conversion IC (or a radio frequency IC), thelocal oscillator 206 is implemented as a separate circuit component andthe COFDM demodulator 210 is implemented in another integrated circuitwhich may be called a demodulator IC. The demodulator IC may have adigital signal processor (DSP) for signal processing.

[0060] In another implementation also the local oscillator 206 isintegrated in the direct conversion IC.

[0061] According to the foregoing description, the channel estimationand correction block 335 of the OFDM demodulator block 302 determines,based on particular pilot signals, a transfer function H(f) of the DVB-Tchannel in use, and based on the transfer function H(f), corrects theeffects that the transmission path causes to the DVB-T signal. The pilotsignals are signals which are transmitted in the DVB-T signal, theirtransmission amplitudes and location in the spectrum being known, inadvance, to the DVB-T receiver.

[0062] There are three different types of pilot signals in the DVB-Tsignal: continual pilot signals, scattered pilot signals and TPS(Transmitter Parameter Signalling) pilot signals. The pilot signals aremodulated onto pilot carriers in the DVB-T transmitter and they form apart of the transmitted DVB-T signal. The COFDM modulated DVB-T signalthus comprises both data carriers (carriers carrying digital data) andpilot carriers (carriers carrying pilot signals). Since the pilotsignals form a part of the DVB-T signal they are processed in a similarmanner as the data carriers in the direct conversion IC.

[0063] In the demodulator IC the continual and scattered pilot signalsare used in channel estimation and correction. In other words, thechannel estimation and correction block 335 determines the transferfunction H(f) based on these pilot signals and corrects the effects,that is distortion, that the transmission path causes to the DVB-Tsignal by multiplicating the DVB-T signal with a function 1/H(f) whichis an inverse function to the determined transfer function H(f). TPSpilot signals are used for signalling particular transmitter parametres.

[0064] In addition to the estimation and correction of the transmissionpath effects the channel estimation and correction block 335 corrects inthe preferred embodiment the IQ-imbalance (or more closely IQ-phaseimbalance or in other words phase error) generated in the DVB-T receiver200. As hinted in the foregoing, IQ-imbalance typically occurs if thephase shift that the adjustable phase shifter 207 performs for the localoscillator signal differs from 90 degrees. The phase difference of theI-signal component and the Q-signal component then, after the mixing,ends up being different from 90 degrees which, in turn, makes thedetection of transmitted bits more difficult. IQ-imbalance may alsooccur if, for example, the propagation times in the I- and Q-branchesdiffer from each other.

[0065] The adjustable phase shifter is typically frequency-dependent andis thus not able to perform a stable 90 degrees phase difference for theI- and Q-branches in the whole wide region of the used frequency band,and that is why the correction is needed. The IQ-imbalance correctionperformed by block 335 is needed for ensuring that the phase differencebetween the signals travelling along the I-branch and the Q-branch is asclose to 90 degrees as possible in the frequency region of the DVB-Tchannel currently in use.

[0066] In the following, the IQ-imbalance correction is described inmore detail. According to the preferred embodiment of the inventionIQ-imbalance is detected from the pilot signals. A correction signal 366is generated in the channel estimation and correction block 335 and itis fed back to the adjustable phase shifter 207 of the direct conversionIC.

[0067] The modulation method that is used in all the pilot signals is2-PSK (Phase Shift Keying). FIG. 4 shows a constellation diagram showingthe constellation points of 2-PSK. It is seen that an ideal 2-PSKmodulated signal does not have a Q-component at all, that is theQ-component is zero. If the Q-component of the received pilot signal isnot zero there exists IQ-imbalance. The IQ-imbalance may caused by thetransmission path, that is the radio channel itself, or by the DVB-Treceiver or both.

[0068] In the preferred embodiment of the invention, the IQ-imbalancecaused by the transmission path is eliminated by averaging. TheIQ-imbalance caused by the transmission path is assumed to be thermal ofits character. That is why the averaging eliminates the IQ-imbalancecaused by the transmission path.

[0069] What is averaged depends on the implementation. In oneimplementation the Q-component of all the continual pilot signals duringone received OFDM symbol (a particular bit sequence) is averaged. Inanother implementation the Q-component of both the continual and thescattered pilot signals is averaged. In yet another implementation theQ-component of both the continual and the scattered as well as the TPSpilot signals is averaged. In yet another implementation the Q-componentof continual and/or scattered and/or TPS pilot signals of more than oneOFDM symbol is averaged. The averaging may be performed to theQ-component residing on the right-hand side half plane of the IQ-planeor to the Q-component residing on the left-hand side half plane of theIQ-plane or both.

[0070] Independent of which of the implementations is used the averagedQ-component represents an estimate of IQ-imbalance generated in theDVB-T receiver 200 (typically most of the IQ-imbalance is generated dueto the non-ideal operation of the adjustable phase shifter 207). Asstated in the foregoing a correction signal 366 is generated in thechannel estimation and correction block 335. The correction signal 366is generated based on the averaged Q-component, that is the Q-componentwhich remains after the averaging has been performed. The correctionsignal 366 which may, for example, be a DC voltage level is fed backfrom the digital domain (every block residing in the demodulator ICafter the analogue-to-digital converters 331 and 332 belong to digitaldomain) to the analogue domain to the adjustable phase shifter 207. Thephase shift of the adjustable phase shifter 207 is corrected with thecorrection signal 366. The purpose is to zero the averaged Q-componentof the pilot signal because if the Q-component gets closer to zero itmeans that the phase difference between the signals travelling along theI- and Q-signal branch gets closer to 90 degrees, that is theIQ-imbalance is corrected or at least reduced in the frequency region ofthe used DVB-T channel.

[0071] The correction signal generation and the correction of theIQ-imbalance may occur iteratively all the time when the DVB-T receiveris receiving the DVB-T signal containing pilot signals.

[0072] Since the IQ-imbalance is according to the preferred embodimentcompletely corrected or at least reduced, the quality of the DVB-Tsignal reception may be improved. If the IQ-imbalance is very severe,for example, 5 degrees or more if the 64-QAM is used, it may bepossible, after the correction, to correctly receive transmitted bits,which without the IQ-imbalance correction would have been wronglyreceived or would not have been received at all.

[0073] In another embodiment of the invention, instead of conveying thelocal oscillator signal to the second input of the second downconversion mixer 205, phase shifting the local oscillator signal in theadjustable phase shifter 207 and conveying the phase shifted oscillatorsignal to the second input of the first down conversion mixer 204, thereare two local oscillators a local oscillator signal of a first localoscillator being conveyed to the second input of the first downconversion mixer 204 and a local oscillator signal of a second localoscillator being conveyed to the second input of the second downconversion mixer 205. A correction signal (or signals) is (are)generated in the channel estimation and correction block 335 and it(they) is (are) fed back to the analogue domain of the DVB-T receiver soas to correct the IQ-imbalance of the DVB-T receiver by adjusting thephase difference between the local oscillator signal of the first localoscillator and the local oscillator signal of the second localoscillator.

[0074]FIG. 5 shows a mobile communication device 80, also referred to asa wireless media terminal, suitable for implementing the invention. Themobile communication device 80 comprises the DVB-T receiver 200 with aninternal integrated antenna ANT, a user interface UI and a control unit86. The DVB-T receiver 200 is coupled to the control unit 86 via a firstcontrol/data bus. The user interface UI is coupled to the control unit86 via a second control/data bus. Additionally, for communicationbetween the mobile communication device 80 and a cellular network (notshown), the mobile communication device may comprise a cellular networkinterface 87 which is coupled to the control unit 86 via a thirdcontrol/data bus. The cellular network interface 87 may comprise a radiofrequency transceiver (not shown).

[0075] The control unit 86 comprises a processor with a memory andsoftware for controlling the operation of the mobile communicationdevice 80. The software contains an MPEG-2 protocol stack for decodingthe demodulated digital data which the DVB-T receiver 200 provides forthe control unit 86 via the first control/data bus. The user interfaceUI comprises a display and a keyboard for enabling the user to use themobile communication device 80. The control unit 86 controls thepresentation of information on the user interface UI, for example thepresentation of the decoded MPEG-2 digital data (video stream) on thedisplay. The demodulated digital data may contain IP (Internet Protocol)packets of an IP datacast or multicast service. The software of thecontrol unit 86 may contain an IP protocol stack to handle the IPpackets.

[0076] If it is desired that the mobile communication device 80 issmall-sized, the small-sized DVB-T direct conversion receiver 200,according to the preferred embodiment of the invention, helps, in manycases, to reduce the size of the mobile communication device 80.

[0077] The present invention enables IQ-imbalance correction in a directconversion receiver. The IQ-imbalance is completely corrected or atleast reduced already in the analogue domain with the aid of acorrection signal generated in the digital demodulator 210 in thedigital domain. When the IQ-imbalance is already corrected in theanalogue domain no additional IQ-imbalance correction is needed in thedigital domain (or even if an additional IQ-imbalance correction wouldstill be performed in the digital domain the algorithms used in it can,in many cases, be made simpler).

[0078] By correcting the IQ-imbalance the signal acquisition may beperformed more easily. When the IQ-imbalance is corrected according tothe invention the probability that the constellation points of thecomplicated phase and amplitude sensitive QAM modulation, used in datacarriers, are detected correctly, in the direct conversion receiver, maybe improved. This means that also the transmitted digital data may bemore accurately regenerated in the direct conversion receiver.

[0079] Although the DVB-T system has been used as an example theinvention is applicable also in other OFDM multicarrier modulation basedsystems, such as the Japanese ISDB-T (Integrated Services DigitalBroadcasting-Terrestrial) system. The invention is also applicable withappropriate changes in other wideband/broadband as well as narrowbandsystems in which IQ-modulation and pilot signals are used.

[0080] The invention is not restricted to mobile devices but it may alsobe used in a fixed DVB-T receiver. This may be present in a so-calledset top box.

[0081] Particular implementations and embodiments of the invention havebeen described. It is clear to a person skilled in the art that theinvention is not restricted to details of the embodiments presentedabove, but that it can be implemented in other embodiments usingequivalent means without deviating from the characteristics of theinvention. The scope of the invention is only restricted by the attachedpatent claims.

1. A method for correcting an IQ-imbalance (In-phase and Quadrature) ofan IQ-based direct conversion receiver (200) the method comprising:receiving a radio frequency signal in the direct conversion receiver(200); conveying, in an analogue domain of the direct conversionreceiver, the received radio frequency signal to an in-phase branch ofthe direct conversion receiver and to a quadrature-phase branch of thedirect conversion receiver; mixing, in the analogue domain of the directconversion receiver, the radio frequency signal of the in-phase branchwith a first mixing signal to form a baseband in-phase (I) signalcomponent and mixing the radio frequency signal of the quadrature-phasebranch with a second mixing signal to form a baseband quadrature-phase(Q) signal component; conveying the baseband in-phase (I) signalcomponent and the baseband quadrature-phase (Q) signal component to adigital demodulator (210); detecting, in the digital demodulator (210),the IQ-imbalance of the direct conversion receiver (200) by analysing atleast one of the baseband in-phase (I) and the quadrature-phase (Q)signal components; correcting the IQ-imbalance, detected in the digitaldemodulator (210), in the analogue domain of the direct conversionreceiver (200) to achieve a 90 degrees phase difference between a futurebaseband in-phase signal component and a future basebandquadrature-phase signal component.
 2. A method according to claim 1,wherein the received radio frequency signal comprises a group of pilotsignals, the method comprising: detecting the IQ-imbalance based on thegroup of pilot signals.
 3. A method according to claim 2, wherein themethod comprises: detecting the IQ-imbalance by analysing at least oneof the baseband in-phase (I) and the quadrature-phase (Q) signalcomponents of the group of pilot signals.
 4. A method according to claim2 or 3, wherein the method comprises: averaging the basebandquadrature-phase (Q) signal component of the group of pilot signals;detecting the IQ-imbalance by finding out whether the averaged basebandquadrature-phase (Q) signal component of the group of pilot signals hasthe value of zero.
 5. A method according to claim 1, wherein the methodcomprises: analogue-to-digital converting the baseband in-phase (I)signal component and the baseband quadrature-phase (Q) signal componentto form a digital baseband in-phase signal component and a digitalbaseband quadrature-phase signal component; detecting the IQ-imbalanceof the direct conversion receiver (200) by analysing at least one of thedigital baseband in-phase and the quadrature-phase signal components. 6.A method according to claim 1, wherein the first mixing signal is alocal oscillator signal and the second mixing signal is a phase shiftedlocal oscillator signal.
 7. A method according to claim 1, wherein thesecond mixing signal is substantially in a 90 degrees phase shiftcompared to the first mixing signal.
 8. A method according to claim 1,wherein the second mixing signal is generated from the first mixingsignal by shifting the phase of the first mixing signal in an adjustablephase shifter (207).
 9. A method according to claim 8, wherein themethod comprises: generating within the digital demodulator (210), basedon the detected IQ-imbalance, a correction signal (366); and correctingthe phase shift of the adjustable phase shifter (207) with the aid ofthe correction signal so as to correct the IQ-imbalance of the directconversion receiver.
 10. A method according to any of the precedingclaims, wherein the direct conversion receiver is a broadband receiverlocated in a mobile communication device (80) so as to form a portablehand-held device.
 11. A method according to any of the preceding claims,wherein the direct conversion receiver is one of the following: a DVB-T(Digital Video Broadcasting-Terrestrial) direct conversion receiver, anISDB-T (Integrated Services Digital Broadcasting-Terrestrial) directconversion receiver.
 12. An IQ-based (In-phase and Quadrature) directconversion receiver (200) for correcting an IQ-imbalance the directconversion receiver comprising: a radio frequency part (201-209) forreceiving a radio frequency signal in the direct conversion receiver(200); an in-phase branch and a quadrature-phase branch, in an analoguedomain (201-209) of the direct conversion receiver (200), for conveyingthe received radio frequency signal in the in-phase branch and thequadrature-phase branch; a first mixer for mixing, in the analoguedomain of the direct conversion receiver, the radio frequency signal ofthe in-phase branch with a first mixing signal to form a basebandin-phase (I) signal component and second mixer for mixing, in theanalogue domain of the direct conversion receiver, the radio frequencysignal of the quadrature-phase branch with a second mixing signal toform a baseband quadrature-phase (Q) signal component; a digitaldemodulator (210) adapted to receive the baseband in-phase (I) signalcomponent and the baseband quadrature-phase (Q) signal component, thedigital demodulator being adapted to detect the IQ-imbalance of thedirect conversion receiver (200) by analysing at least one of thebaseband in-phase (I) and the quadrature-phase (Q) signal component;means (207, 335, 366) for correcting the IQ-imbalance, detected in thedigital demodulator (210), in the analogue domain of the directconversion receiver (200) to achieve a 90 degrees phase differencebetween a future baseband in-phase signal component and a futurebaseband quadrature-phase signal component.
 13. A communication device(80) comprising an IQ-based (In-phase and Quadrature) direct conversionreceiver (200) for correcting an IQ-imbalance the direct conversionreceiver comprising: a radio frequency part (201-209) for receiving aradio frequency signal in the direct conversion receiver (200); anin-phase branch and a quadrature-phase branch, in an analogue domain(201-209) of the direct conversion receiver (200), for conveying thereceived radio frequency signal in the in-phase branch and thequadrature-phase branch; a first mixer for mixing, in the analoguedomain of the direct conversion receiver, the radio frequency signal ofthe in-phase branch with a first mixing signal to form a basebandin-phase (I) signal component and second mixer for mixing, in theanalogue domain of the direct conversion receiver, the radio frequencysignal of the quadrature-phase branch with a second mixing signal toform a baseband quadrature-phase (Q) signal component; a digitaldemodulator (210) adapted to receive the baseband in-phase (I) signalcomponent and the baseband quadrature-phase (Q) signal component, thedigital demodulator being adapted to detect the IQ-imbalance of thedirect conversion receiver (200) by analysing at least one of thebaseband in-phase (I) and the quadrature-phase (Q) signal component;means (207, 335, 366) for correcting the IQ-imbalance, detected in thedigital demodulator (210), in the analogue domain of the directconversion receiver (200) to achieve a 90 degrees phase differencebetween a future baseband in-phase signal component and a futurebaseband quadrature-phase signal component.
 14. A communication device(80) according to claim 13, wherein the communication device comprises,in addition to the direct conversion receiver (200), a cellular networkinterface (87) for communicating information with a cellular network.15. A system comprising a transmitter and an IQ-based (In-phase andQuadrature) direct conversion receiver (200) for correcting anIQ-imbalance, the transmitter comprising a modulator for transmitting aradio frequency signal to the direct conversion receiver the directconversion receiver comprising: a radio frequency part (201-209) forreceiving the radio frequency signal in the direct conversion receiver(200); an in-phase branch and a quadrature-phase branch, in an analoguedomain (201-209) of the direct conversion receiver (200), for conveyingthe received radio frequency signal in the in-phase branch and thequadrature-phase branch; a first mixer for mixing, in the analoguedomain of the direct conversion receiver, the radio frequency signal ofthe in-phase branch with a first mixing signal to form a basebandin-phase (I) signal component and second mixer for mixing, in theanalogue domain of the direct conversion receiver, the radio frequencysignal of the quadrature-phase branch with a second mixing signal toform a baseband quadrature-phase (Q) signal component; a digitaldemodulator (210) adapted to receive the baseband in-phase (1) signalcomponent and the baseband quadrature-phase (Q) signal component, thedigital demodulator being adapted to, detect the IQ-imbalance of thedirect conversion receiver (200) by analysing at least one of thebaseband in-phase (I) and the quadrature-phase (Q) signal component;means (207, 335, 366) for correcting the IQ-imbalance, detected in thedigital demodulator (210), in the analogue domain of the directconversion receiver (200) to achieve a 90 degrees phase differencebetween a future baseband in-phase signal component and a futurebaseband quadrature-phase signal component.